This invention relates to a light-conducting plate for a planar light emitting device which can be used in displays, especially those of the types which do not emit light by themselves but utilize light from an external unit, and a method of producing the light-conducting plate. The light-conducting plate of the invention is useful for planar light emitting devices, such as a display panel of meters, etc. on the dashboard and a back light or a front light of liquid crystal displays (LCDs).
Brightness and color of a display of LCDs, etc. are governed by a planar light emitting device used. The planar light emitting device is therefore required to emit bright and uniform light.
Conventional planar light emitting devices are assembled from various constituting members including, as shown in FIG. 1, a light source comprising a lamp 1 and a reflector 2 and a light-conducting plate 5 having a reflecting layer 3 and a reflecting sheet 4. The light-conducting plate 5 propagates the light from the light source incident on its side surface and emits the propagated light from its light-emitting surface. If necessary, the device is provided with a light-scattering member 6 for non-directionally scattering the light from the light-conducting plate 5 and the device generally also includes one or two output light controlling member(s) 7 having a prism lens shape.
Conventional studies for obtaining bright and uniform light from the planar light emitting device had been focused, for the part, on the light source (the lamp 1 and the reflector 2), the material or the pattern of the reflecting layer 3, the material and configuration of the light-scattering member 6 or the output light controlling member 7, or the number of the constituent members to be stacked. Notwithstanding the fact that the brightness (luminance) or uniformity of the brightness largely depends on the light-conducting plate 5, little study has been given to the light-conducting plate itself. JP-U-B-58-46447 (The term xe2x80x9cJP-U-Bxe2x80x9d used herein means a xe2x80x9cpublished Japanese Utility Modelxe2x80x9d) proposes dispersing light-scattering particles in a light-conducting plate, aiming to efficiently scatter the incident light by the particles to improve the brightness (luminance) of the emitted light. According to the disclosure, however, the concentration of the particles is so complicatedly related to the size of a light-conducting plate such that one concentration is set for one size of a light-conducting plate, and not generally used for every size of the light-conducting plate. JP-A-3-209293 (The term xe2x80x9cJP-Axe2x80x9d used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses incorporation into a light-conducting plate air bubbles which are expected to produce the same effect as by the light-scattering particles. However, because the air bubbles are as large as 10 to 2000 xcexcm and meet difficulty in controlling the disperse state, making it hard to prepare a trial product in a laboratory, still more to manufacture on an large scale. JP-A-10-138275 proposes a method of producing a light-conducting plate by injection molding in which a mold cavity is previously filled with carbon dioxide to a pressure of 10 Mpa or less. This method aims at elimination of product defects through improvement in precision of transferring fine unevenness of the cavity wall onto the light-conducting plate thereby to improve the performance of the light-conducting plate. WO98/52734 discloses a method of producing a light-conducting plate, in which a molten resin having dissolved therein 0.2% by weight or more of gas so as to have a reduced melt viscosity is injection molded in a cavity having previously been pressurized with gas at or above a certain pressure at which no foaming is induced at the flow front. The method also aims at improvement of performance by eliminating product defects such that the such that the reproducibility of the cavity configuration, such as sharp edges and fine unevenness, may be improved, weld lines may be less noticeable, and the flowability of the resin may be improved to reduce the resin orientation or the birefringence. Neither JP-A-10-138275 nor WO98/52734 clarifies the relationship between the gas content in the molded product and the performance as a light-conducting plate.
An object of the present invention is to provide a light-conducting plate which emits light with improved brightness (luminance), improved uniformity of the luminance, and reduced unevenness of color, irrespective of its size and shape.
Another object of the present invention is to provide a method of manufacturing the light-conducting plate.
Other objects and effects of the present invention will become apparent from the following description.
As a result of extensive studies, the present inventors have found that the above objects of the present invention are accomplished by the presence of carbonic acid gas in the light-conducting plate in a concentration of 0.01 to 2% by weight.
That is, the above objects of the present invention have been achieved by providing the following light-conducting plates and method of producing the same.
(1) A light-conducting plate which comprises a transparent resin containing a carbonic acid gas and which has a carbonic acid gas content of from 0.01 to 2% by weight based on the weight of the light-conducting plate.
(2) The light-conducting plate according to the above item (1), wherein said transparent resin is selected from methacrylic resins, polycarbonate resins and cyclic olefin resins.
(3) The light-conducting plate according to the above item (1) or (2), wherein said transparent resin is a polycarbonate resin having a terminal hydroxyl group in a proportion of from 1 to 30 mol % based on the whole terminal groups.
(4) The light-conducting plate according to the above item (1) or (2), wherein said transparent resin contains at least one of a saturated aliphatic higher alcohol, a glycerol higher fatty acid ester, and an ultraviolet absorber.
(5) The light-conducting-plate according to any one of the above items (1), (2) and (4), wherein said transparent resin is a methacrylic resin.
(6) The light-conducting plate according to any one of the above items (1) to (5), wherein said transparent resin further contains fine particles.
(7) The light-conducting plate according to any one of the above items (1) to (6), which has a light-emitting surface and an opposite surface and has fine unevenness on at least one of these surfaces.
(8) The light-conducting plate according to any one of the above items (1) to (7), which has a light-emitting surface having a lens shape.
(9) The light-conducting plate according to any one of the above items (1) to (8), which has a wedge shape having an inclined reflecting plane.
(10) A method of producing a light-conducting plate, which comprises:
melt-kneading a transparent resin in a cylinder while pressurizing the cylinder by feeding a carbonic acid gas thereinto under a pressure of from 1 to 15 MPa;
injecting the molten resin into a mold cavity pressurized with a carbonic acid gas beforehand to a pressure which is from 1 to 15 MPa and which is at least a pressure where foaming does not occur at a flow front of the molten resin to be injected;
releasing part of the carbonic acid gas by partially reducing the pressure in the cavity while conducting dwelling; and
cooling the molded resin to obtain a molded article,
wherein said molded article contains a carbonic acid gas and having a carbonic acid gas content of from 0.01 to 2% by weight based on the weight of the molded article.